The invention concerns a nuclear magnetic resonance (NMR) tomograph with a superconducting main field coil for the production of a static, homogeneous magnetic field in an investigational volume of the magnetic resonance imaging system whose center coincides with the center of a coordinate origin of a cartesian x-, y-, z-coordinate system, having a pair of mutually similar outer field coils which are arranged on a common axis (z) at an axial separation (g.sub.1) with respect to each other as well as a pair of inner likewise mutually similar field coils coaxial to the outer field coils, whereby both coils pairs are arranged symmetrically with respect to a central middle plane (E) which runs perpendicular to the common axis (z), whereby the axial separation (g.sub.1) of the outer field coils assumes a value between 1/4 and 3/4, preferentially 1/2 of the inner diameter (d.sub.a2) of the outer field coils, whereby the axial separation (g.sub.2) of the inner field coils is slightly, e.g. up to 15%, different than the axial separation (g.sub.1) of the outer field coils, whereby, during operation of the main field coil, the current flow direction in the inner field coils is oppositely directed to that in the outer field coils, with an axial room temperature bore extending in the direction of the homogeneous magnetic field and a transverse access opening extending transversely to the direction of the homogeneous magnetic field and having a device for the compensation of inhomogeneities in the static magnetic field.
An NMR tomograph of this kind is known the art, for example, from DE 39 07 927 A1.
The main field coil of the tomograph which is known in the art consists of a double Helmholtz-coil configuration with opposite current directions. An NMR tomography magnetic system of this type, having relatively small axial and radial dimensions as well as relatively low overall weight, facilitates a free access to the investigational volume from a plurality of different directions, whereby the configuration is also relatively insensitive to eddy currents which can occur during switching of the field gradients necessary for tomography. Transverse access openings to the investigational volume are particularly important since claustrophobia problems are strongly reduced or do not occur for the patient being examined in the tomograph. In addition, a plurality of therapeutic measures, for example surgery, in particular microsurgery or irradiation, which can be directly monitored and checked with a tomography apparatus, require as free an axial and transverse access to the patient as possible.
On the other hand, the NMR slice images which are created with a therapeutic tomograph of this type should have as high a resolution and as high a signal-to-noise ratio as possible. For this reason it is necessary for the static magnetic field produced by the main field coil in the investigational volume to have as high a homogeneity as possible.
A useful mathematical tool for the description of inhomogeneities in a region about the symmetry center of a magnetic field is given by the expansion of the field in spherical harmonics. In NMR tomographs only the axial, e.g. the z-components dominate. In general one has: ##EQU1## In ideal rotationally symmetric configurations all terms in the sum vanish which are associated with non-rotationally symmetric field components, e.g. all terms with m not equal to 0. In the usual notation these types of terms are designated "tesseral" whereas terms having m=0 are called "zonal".
Furthermore with a mirror-symmetric configuration all terms except those with amplitudes A.sub.n0 having odd values of n vanish. In addition with coil configurations associated with the above cited DE 39 07 927 A1, all terms with n&lt;8 vanish. The following expression then results for the homogeneous magnetic field in the z-direction: ##EQU2## In particular along the z-axis one has for the double Helmholtz-configuration with oppositely directed currents EQU B.sub.z =A.sub.00 +A.sub.80 z.sup.8 +A.sub.10,0 z.sup.10 + (3)
A.sub.00 is the desired homogeneous field. The remaining terms represent field distortions which are unavoidable with this coil configuration even in the event of a perfect construction which at z- or r-values of 20 cm from the symmetry center have relative, e.g. relative to A.sub.00, magnitudes of approximately 10 ppm. For smaller z- or r-values these distortions are generally much smaller due to the z.sup.n -dependence (n.gtoreq.8) so that, within a spherical volume about the symmetry center with a diameter of approximately 40 cm, only field distortions below 10 ppm occur and high quality NMR imaging methods are thereby possible.
Due to mechanical tolerances during the manufacture of the coil bodies as well as to the in principle helical instead of ideally rotationally symmetrical shape of the windings of the main field coil, one obtains deviations from this ideal behavior which can result in non-vanishing distortion amplitudes A.sub.nm, B.sub.nm with n&lt;8 and m.gtoreq.0. In practice one must experimentally determine these distortion amplitudes through measurements and compensate for them with the assistance of superconducting or resistive auxiliary coils or with strategically and advantageously placed pieces of iron, for example in the bore of the magnet coil.
A compensation of this kind is known in the art as "shiming". For example, superconducting shim-coils are described in DE 35 11 303 A1. The utilization of ferromagnetic shim plates is known in the art from DE 17 64 564 A1.
With a therapeutic tomograph having sidewardly and axially open coil systems facilitating both an axial as well as a transverse access to the investigational volume, there is the additional difficulty compared to conventional tomographs that there is neither a bore for securing iron pieces nor another support body for attaching superconducting or resistive correction coils in the entire region of the sideward access opening. Substantial limitations therefore obtain for the placing of shim elements. Certain types of field distortions can therefore no longer be easily compensated for. The preferred locations in conventional systems for these types of shim elements, in the middle region near to the homogeneity volume, are located, in the described therapy systems, in the vicinity of the transverse access and therefore must remain free.
Furthermore it is necessary, with all superconducting correction coils, to prevent inductive coupling to the superconducting main field coil. Otherwise, for example, a time dependent drift of the homogeneous magnetic field produced by the main field coil would lead to a discharge of the correction coils.
It is therefore the purpose of the present invention to introduce an NMR tomograph of the above mentioned kind with which homogeneity of the magnetic field is facilitated without hindering the axial or transverse access to the investigational volume.